Manufacturing method of semiconductor laser with non-absorbing mirror structure

ABSTRACT

An InGaAlP NAM structure laser is formed with a double-heterostructure section disposed on an n-type GaAs substrate. The double-heterostructure section includes a first cladding layer of n-type InGaAlP, a non-doped InGaP active layer, and a second cladding layer of p-type InGaAlP. An n-type GaAs current-blocking layer having a stripe opening and a p-type GaAs contact layer are sequentially formed on the second cladding layer by MOCVD crystal growth. A low-energy band gap region is defined in a central region of the active layer located immediately below the stripe opening. A high-energy band gap region is defined in a peripheral region of the active layer corresponding to a light output end portion of the laser and located immediately below the current-blocking layer. Therefore, self absorption of an oscillated laser beam at the output end portion can be reduced or prevented.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to semiconductor laser devices using acompound semiconductor material and, more particularly, to a method ofmanufacturing semiconductor laser devices for stably providing a laseroscillation of visible light at room temperature.

2. Description of the Related Art

Rapid advances in semiconductor technologies have led to a variety ofapplications for semiconductor devices serving as solid-statelight-emitting elements. Unceasing efforts have been made to satisfyneeds for higher outputs and higher reliability. In recent years,indium/gallium/aluminum/phosphorus (to be referred to as "InGaAlP"hereinafter)-based semiconductor materials have received a great deal ofattention as materials for semiconductor lasers which provide visiblelight oscillation. This is because the compound semiconductor materialshave largest band gaps among Group III-V compound semiconductor mixedcrystals except for nitrides.

In particular, a semiconductor laser with a double-heterostructurehaving active and cladding layer of InGaAlP on a gallium arsenide (to bereferred to as "GaAs" hereinafter) substrate can exhibit a stable laseroscillation of visible light in a 0.6-micrometer band at roomtemperature. A semiconductor laser of this type is expected to be usedin various applications which cannot be realized using conventionalinfrared-range semiconductor lasers. Therefore, a semiconductor laser ofthis type is a very expecting light-emitting device.

Presently available semiconductor laser devices having an InGaAlPdouble-heterostructure, however, suffer from the followingdisadvantages: (1) a light output cannot be increased up to a desiredvalue; and (2) it is difficult to keep satisfactory operationreliability in a maximum light output oscillation mode.

According to the studies by the present inventors, it was confirmed thatthe above problems were mainly caused by undesired self light absorptionat a light output end of an InGaAlP active layer in a laser oscillationmode, as will be explained hereinafter in more detail.

In general, a stripe light guide structure is applied to an InGaAlPsemiconductor laser. A GaAs current-blocking layer is formed on adouble-heterostructure in which an InGaAlP active layer is sandwiched byupper and lower cladding layers, and the current-blocking layer isprovided with an elongated groove, i.e., a stripe opening. A GaAs ohmiccontact layer covers the channel and the current-blocking layer. In anoscillation mode, oscillated laser beams are confined in only theopening, and hence this opening serves as a light waveguide channel. Anoscillation wavelength is determined depending on a band gap energy ofthe InGaAlP layer serving as a light-emitting region of the activelayer. An InGaAlP active layer is conventionally formed using anepitaxial growth method which is popular among persons skilled in theart. Therefore, a band gap energy is simply constant across the entireactive layer.

With such an arrangement, when supply of a maximum injection current iscontinued in order to successively perform an oscillation at a maximumlight output level of the semiconductor laser at room temperature, thelight output is abruptly decreased, and operation stability is damaged.Such degradation in performance is also observed in semiconductor lasersof other types such as a transverse mode stabilized InGaAlP laser.Therefore, the above-mentioned degradation in performance is an inherentphenomenon of InGaAlP compound semiconductor materials, and can beconsidered to be caused by its limited inherent allowable light density.In practice, substrates of several elements degraded due to laseroscillation were removed, and a current injection light-emitting patternwas observed from the substrate side. It was confirmed that a blackportion was formed near each light output end of the laser element. Thisobservation result demonstrates generation of the following viciouscircle. That is, a light density of the laser beam output end of theInGaAlP active layer is increased beyond an inherent allowable limit ofthe material, self light absorption is accelerated, heat is generated,and the heat generation causes further acceleration of self lightabsorption. Such a vicious circle of self light absorption adverselyaffects, i.e., not only decreases a laser oscillation efficiency, butalso physically breaks down a light output layer in the active layer,since it causes a damage, melting, and degradation of the layer qualityof the layers near the laser oscillation output end in the active layer.

These problems have spoiled usefulness and future applications ofInGaAlP semiconductor laser devices and are decisive for semiconductormanufacturers. Applications of InGaAlP laser devices without drasticresolutions for the above problems seem impossible.

In order to prevent degradation in performance associated with theabove-mentioned self light absorption, several high-output semiconductorlaser devices each having a "non-absorbing mirror" structure have beenproposed. For example, a self-absorption prevention technique isdisclosed in "An AlGaAs Window Structure Laser", HIRO O. YONEZU et al,IEEE Journal of Quantum Electronics, Vol. QE-15, No. 8, August, 1979 atpp. 775-781, wherein impurity such as Zn is doped into an active layerin such a manner that the both end portions thereof are kept undoped, sothat the band gap energy at the both end portion of the active layer ishigher than that of the remaining portions thereof. Another technique isdisclosed in "AlGaAs Window Stripe Buried Multiquantum Well Lasers",HISAO NAKASHIMA et al, Japanese Journal of Applied Physics, Vol. 24, No.8, August, 1985, at pp. L647-L649, wherein an impurity such as Zn isdiffused in two end portions of the active layer to increase itsinternal band gap energy in only the diffusion region, thus decreasingself light absorption. According to the technique disclosed in thisarticle, however, a method of manufacturing lasers with a "non-absorbingmirror" structure requires very complicated manufacturing processesinvolving strict control of crystal growth. Therefore, practicalapplication of these processes cannot be expected for semiconductormanufacturers very much. In addition, application of these techniques tothe manufacture of high-power semiconductor lasers using a specificInGaAlP-based semiconductor material, which suffers from more difficultcrystal growth manufacture than GaAlAs, can be hardly expected.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a new andimproved a manufacturing method of a high-power semiconductor laser ofNAM structure.

In accordance with the above object, the present invention is addressedto a specific semiconductor laser which comprises a first cladding layerformed above a semiconductor substrate, an active layer disposed on thefirst cladding layer, and a second cladding layer provided on the activelayer. The second cladding layer includes a waveguide channel region towhich a current is mainly supplied in an oscillation mode of the laser,and an output end from which an oscillated laser beam is externallyoutput. A current-blocking layer is disposed on the second claddinglayer, includes an opening defined above a central region of thewaveguide channel region, and covers the second cladding layer in onlyits peripheral portion corresponding to the output end. The active layerincludes a high-energy band gap region in a peripheral portion thereofadjacent to the output end of the second cladding layer, and a band gapenergy of portions except for the high-energy band gap region is keptlow, whereby self absorption of the oscillated laser beam at the outputend can be reduced or prevented.

According to the manufacturing method of this invention, in order toform such a high-energy band gap region in the active layer, additionalsteps such as the step of selectively doping an impurity are notrequired at all. A contact layer may be formed on the current blockinglayer having the opening by a metal organic chemical vapor deposition(MOCVD) method. At this time an impurity is naturally diffused from thesecond cladding layer located immediately under the current-blockinglayer to the active layer, and hence the high-energy band gap region canbe defined at only a peripheral portion of the active layer

The present invention, and its objects and advantages will become moreapparent in the detailed description of preferred embodiments presentedbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

In the detailed description of preferred embodiments of the presentinvention presented below, reference is made to the accompanyingdrawings of which:

FIG. 1 is a schematic diagram showing the entire arrangement of ahigh-output InGaAlP laser with a NAM structure according to a preferredembodiment of the present invention;

FIG. 2 is a diagram showing a plan view of a current-blocking layerhaving a stripe opening of the laser shown in FIG. 1;

FIG. 3 is a diagram showing a cross-sectional structure taken along theline III--III of the laser shown in FIG. 1;

FIG. 4 is a diagram showing a cross-sectional structure taken along theline IV--IV of the laser shown in FIG. 1;

FIGS. 5A to 5C are diagrams showing, in schematic cross-section, some ofthe major steps in the formation of the laser in accordance with onepreferred embodiment of the present invention;

FIG. 6 is a schematic diagram showing the entire arrangement of ahigh-output InGaAlP laser with a NAM structure according to anotherembodiment of the present invention;

FIG. 7 is a diagram showing a plan view of a current-blocking layerhaving a stripe opening of the laser shown in FIG. 6;

FIG. 8 is a diagram showing a cross-sectional structure taken along theline VIII--VIII of the laser shown in FIG. 6;

FIG. 9 is a diagram showing a cross-sectional structure taken along theline IX--IX of the laser shown in FIG. 6;

FIG. 10 is a schematic diagram showing the entire arrangement of ahigh-output InGaAlP laser with a NAM structure according to stillanother embodiment of the present invention;

FIGS. 11A through 11D are diagrams showing, in schematic cross-section,some of the major steps in the formation of the laser shown in FIG. 10in accordance with the present invention;

FIG. 12 is a schematic diagram showing the entire arrangement of ahigh-output InGaAlP laser with a NAM 7 structure according to stillanother embodiment of the present invention;

FIG. 13 is a graph showing a measurement result of changecharacteristics of a temperature increase ΔT of an active layer as afunction of a value Ls in a continuous oscillation mode of the lasershown in FIG. 12; and

FIG. 14 is a graph showing correlation characteristics obtained byplotting measurement values of a substrate length Ls as a function of achannel width Ws when a target value of the temperature increase ΔT ofthe laser is 10 degrees.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, a semiconductor laser according to a preferred embodiment ofthe present invention is generally designated by reference numeral "10".A semiconductor substrate 12 is a gallium arsenide (GaAs) layer ofn-type conductivity. A buffer layer 14 is formed on the n-GaAs substrate12 by well-known crystal growth. The buffer layer 14 may be a GaAslayer, typically 0.5 micrometers thick and doped with an n-type impurityto a concentration of 8×10¹⁷ atoms per cubic centimeter.

A double-heterostructure section is disposed on a surface of the n-GaAsbuffer layer 14. The double-heterostructure section includes a firstcladding layer 16, an active layer 18, and a second cladding layer 20.The first cladding layer 16 may be an n-type In₀.5 Ga₀.15 Al₀.35 P layerhaving a thickness of approximately 0.8 micrometers, doped with ann-type impurity such as Si to a concentration of 4×10¹⁷ atoms per cubiccentimeter. The active layer 18 may be a nondoped In₀.5 Ga₀.5 P layer ofp-type conductivity, typically 0.06 micrometers thick. In principle, acarrier concentration of the active layer 18 is 1×10¹⁷ per cubiccentimeter or less. The active layer 18 may be of an n- or p-type. Thesecond cladding layer 20 may be a p-type In₀.5 Ga₀.15 Al₀.35 P layerhaving a thickness of approximately 0.8 micrometers, doped with a p-typeimpurity such as Zn to a concentration of 4×10¹⁷ atoms per cubiccentimeter. With such an arrangement, it was confirmed based on X-raydiffraction measurement by the present inventors that thedouble-heterostructure section was excellently lattice-matched with then-GaAs substrate 12.

A p-type cap layer 22 is disposed on the second cladding layer 20 to apredetermined thickness, for example, 0.05 micrometers. The cap layer 22may be an In₀.5 Ga₀.5 P layer that is doped with a p-type impurity suchas Zn to a concentration of 2×10¹⁸ atoms per cubic centimeter. An n-typecurrent-blocking layer 24 is positioned on the cap layer 22. Thecurrent-blocking layer 24 may be a GaAs layer, typically 0.5 micrometersthick and doped with an n-type impurity such as Si to a concentration of2×10¹⁸ atoms per cubic centimeter.

As shown in FIG. 2, the current-blocking layer 24 has a rectangularelongated opening 26 in its central region. In this description, theopening 26 may often be referred to as a "stripe opening" hereinafter.The top surface of the underlying cap layer 22 is partially exposedthrough the opening 26. The longitudinal direction of the opening 26corresponds to a direction of laser oscillation (waveguide direction) ofthe double-heterostructure semiconductor laser 10. Assume that awaveguide channel section is a region defined between parallel brokenlines 28a and 28b, as shown in FIG. 2. Therefore, a region 30a or 30bhatched for the sake of convenience serves as an output end of anoscillated laser beam. As is apparent from FIG. 2, the stripe opening 26of the current-blocking layer 24 does not reach the light output endregion. In this embodiment, the opening 26 is terminated at a positionspaced apart inward from the end faces of the regions 30a and 30b of thelaser 10 by only approximately 5 micrometers. In other words, each ofboth ends 26a and 26b of the opening 26 is located to be spaced apartfrom the corresponding end face of the regions 30a and 30b of the laser10 by 5 micrometers (or more). This separation distance is designated by"d" in FIG. 2. An ohmic contact layer 32 (see FIG. 1) is formed on thecurrent-blocking layer 24 to bury the opening 26. The ohmic contactlayer 32 may be a p-type GaAs layer, approximately 3 micrometers thickand doped with a p-type impurity such as Zn to a concentration of 3×10¹⁸atoms per cubic centimeter.

The structure of the cross section of the laser 10 is shown in detail inFIGS. 3 and 4. In the active layer 18, low- and high-energy band gapregions 18a and 18b are defined. The low-energy band gap region 18a isformed in a central region of the active layer 18, and is locatedimmediately below the stripe opening 26 of the above-mentionedcurrent-blocking layer 24. A planar shape of the low-energy band gapregion 18a, therefore, corresponds to that of the opening 26. Thelow-energy band gap region 18a serves as a main light-emitting section.The high-energy band gap region 18b surrounds the low-energy band gapregion 18a in the active layer 18. In FIGS. 3 and 4, the region 18b ishatched for the purpose of enhancement of visual distinction. Adifference in energy band gap between these regions 18a and 18b in theactive layer 18 is caused by a difference in atomic alignment ofcrystals. Such a difference in band gap energy realizes a non-absorbingmirror (NAM) structure in the laser 10. Therefore, oscillation laserbeams generated in the active layer 18 are efficiently confined in theactive layer 18, thus minimizing undesirable absorption in the lightoutput end.

It should be noted that the above-mentioned difference in band gapenergy between the regions 18a and 18b is required to be at least 20 meVin order to provide an excellent NAM structure in the laser 10. Whenphotoluminescence evaluation was performed for devices which wereexperimentally manufactured in practice by the present inventors, theband gap energy of the low-energy band gap region 18a was smaller thanthat of the high-energy band gap region 18b by approximately 20 to 90meV. When a transmission electron diffraction image of the high-energyband gap region 18b was examined, it was demonstrated that the region18b in the active layer 18 has a zincblende structure. On the otherhand, excessive spots were observed in the transmission electrondiffraction image of the low-energy band gap region 18a. This factapparently proves the presence of the above-mentioned difference in bandgap energy.

An oscillation wavelength of the double-heterostructure semiconductorlaser 10 with the above arrangement is determined depending on a bandgap energy of the low-energy band gap region 18a of the active layer 18in which electron injection is mainly performed. An absorptioncoefficient of the high-energy band gap region 18b with respect to theoscillation light beam having the determined wavelength is smaller thanthat of the low-energy band gap region 18a. Therefore, undesirable selfabsorption of the oscillation light beam at the low-energy band gapregion 18a (i.e., laser beam output end) which causes a reduction inoscillation efficiency of the laser 10 can be suppressed or prevented.

The experiment performed by the present inventors has demonstrated thefollowing results. With the laser structure shown in FIGS. 1 through 4,when semiconductor lasers each having a width of the stripe opening 26of 7 micrometers, a resonance cavity length of 300 micrometers, and adistance d between each of two ends of the opening 26 and thecorresponding laser beam output end of 15 micrometers were subjected tocontinuous oscillation, each threshold current of the lasers wasslightly increased to 75 mA, whereas stability of the oscillationwavelength and basic operation characteristics such as light outputcharacteristics were excellent. In particular, a maximum light outputlevel was as high as 50 mW or more, and was very stable without decreaseeven if oscillation was continued for a long period of time e.g., 1,000hours or more. This indicates that undesirable self absorption at thelight output end in the conventional laser structure is successivelysuppressed. More specifically, in the laser structure according to thepresent invention, a current was forcibly supplied to exceed the maximumlight output level. Also in this case, degradation in oscillationperformance due to an abrupt increase in threshold value which oftenoccurs in the prior art was not observed at all. In the laser accordingto the present invention, it is considered that the maximum light outputlevel is simply determined in accordance with saturation of light outputcaused by heat generation.

A method of manufacturing the device of this embodiment will bedescribed below with reference to FIGS. 5A to 5C. According to thismanufacturing method, the InGaAlP NAM structure lasers can bemanufactured by a metal organic chemical vapor deposition (MOCVD)process which is presently available and is widely performed.

As shown in FIG. 5A, the n-GaAs buffer layer 14 is formed on the galliumarsenide (GaAs) substrate 12 of n-type conductivity. Thedouble-heterostructure section, consisting of the first cladding layer16 of n-type InGaAlP, the non-doped p-type InGaAlP active layer 18, andthe second cladding layer 20 of p-type InGaAlP, is formed on the n-GaAsbuffer layer 14. The p-type InGaP cap layer 22 and the n-type GaAscurrent-blocking layer 24 are sequentially disposed on thedouble-heterostructure section (the current-blocking layer 24 in FIG. 5Ais hatched for the purpose of visual distinction from the remaininglayers). The material and formation conditions (thickness, carrier andimpurity concentrations, and the like) of these layers are describedabove, and a detailed description thereof will be omitted. The layers 12to 24 were formed by the MOCVD method. This step is referred to as"first-step MOCVD process" hereinafter. The InGaAlP layers whichconstitute the first and second cladding layers 16 and 20 each have aproperty of varying a band gap energy depending on a crystal growthmethod and growth conditions while a mol fraction (i.e., composition) inthe grown crystal is constant. It is considered, by persons skilled inthe art, that this property is caused by a change in atomic alignment inthe crystal depending on crystal growth method and growth conditions. Inthe first-step MOCVD process, a surface temperature of the crystal isset to be 800° C. or less; it was set to be, e.g., 730° C. in thisembodiment. Under the condition, the entire active layer 18 had auniform low-energy band gap. In other words, in this manufacturing step,the entire active layer 18 served as a low-energy band gap region. Inpractice, a photoluminescence light-emitting wavelength of the activelayer 18 at this time was 660 nanometers. Note that a concentration ofan impurity, i.e., Zn, doped in the p-type InGaAlP cladding layer(second cladding layer) 20 may fall within the range of 1×10¹⁷ to 5×10¹⁷atoms per cubic centimeter. If the concentration falls within thisrange, satisfactorily excellent temperature characteristics requiredwhen the semiconductor laser is used in practice can be provided.

As shown in FIG. 5B, the current-blocking layer 24 was subjected tochemical etching using a photolithography technique and a sulfuricacid-based etchant to form the stripe opening 26 therein. A planar shapeof the opening 26 is described above with reference to FIG. 2.

As shown in FIG. 5C, the p-type GaAs ohmic contact layer 32 wascrystal-grown on the top surface of the resultant structure by the MOCVDmethod. It should be noted that this step is to be referred to as a"second-step MOCVD process" hereinafter. At this time, the temperatureof a crystal surface is set to be lower than that in the abovefirst-step MOCVD process. In this embodiment, the temperature of thecrystal surface in the second-step MOCVD process was kept to be 650° C.With such an MOCVD process, the atomic alignment was changed in onlyperipheral regions in the InGaP active layer 18 located immediatelybelow the current-blocking layer 24, and the high-energy band gap region18b was naturally formed. In the remaining central region, i.e., theregion immediately below the strip opening 26, in the active layer 18,such a change in atomic alignment did not occur at all. Therefore, thecentral region still served as the low-energy band gap region 18a. Asdescribed above, the region 18b is substantially self-aligned with thestripe opening 26. The high-energy band gap region 18b isdot-illustrated in FIG. 5C for the purpose of visual distinction fromthe low-energy band gap region 18a. A careful attention must be paid tothe fact that no specific additional processes are required to form thehigh-energy band gap region 18b in the active layer 18, and the region18b is naturally formed by MOCVD crystal growth of the contact layer 32.

The present inventors have made extensive studies on a physicalmechanism which causes such a natural formation phenomenon of thehigh-energy band gap region 18b. As a result, it is assumed that achange in atomic alignment of InGaP which allows formation of the region18b is caused by a change in annealing effect of the active layer 18depending on the presence/absence of the layer component of the GaAscurrent-blocking layer 24 above the active layer 18. When aphotoluminescence light-emitting wavelength in the high-energy band gapregion 18b was measured in the device structure obtained in practice inFIG. 5C, the wavelength was 645 nanometers, and a decrease in wavelengthby approximately 20 meV was recognized. A NAM structure obtained byforming the high-energy band gap region 18b can be manufactured withexcellent reproducibility by appropriately controlling a crystal growthtemperature in the second MOCVD process. The temperature control isrecommended to be performed such that the first MOCVD crystal growthtemperature is set within the range of 730° to 800° C., and the secondMOCVD crystal growth temperature is set to be lower than the above settemperature by 50° to 100° C.

After the laser structure shown in FIG. 5C is obtained, a laser beamoutput end is formed using a well-known conventional technique. Forexample, after electrode layers are respectively formed on the substrate12 and the contact layer 24 in the structure shown in FIG. 5C, acleavage treatment is performed. Therefore, the above structure istreated so that the high-energy band gap region 18b in the active layer18 serves as an oscillation light output end, thus completing theInGaAlP NAM structure laser device. Note that, according to the laserdisclosed in this embodiment, the current-blocking layer 24 alsoprevents self absorption caused by heat generation due to laseroscillation in the light output end serving as a non-excited region.However, this current-blocking layer 24 does not contribute torealization of the above-mentioned NAM structure. Therefore, when theabove function of the current-blocking layer 24 is not particularlyrequired in a given application, the current-blocking layer 24 may beomitted.

FIG. 6 perspectively shows the entire structure of an InGaAlP NAMstructure laser according to another embodiment of the presentinvention, which is generally designated by numeral "50". The same partsin the laser 50 of this embodiment are denoted by the same referencenumerals as in the laser 10 in the above embodiment, and a repetitivedetailed description thereof will be omitted.

A p-type InGaP layer 52 is formed on a second cladding layer 20 whichconstitutes a double-heterostructure section together with a firstcladding layer 16 and an active layer 18. This layer 52 may be typically0.005 micrometers thick and doped with a p-type impurity such as Zn to aconcentration of 1×10¹⁸ atoms per cubic centimeter. This layer 52 servesas an etching stopper. A mesa-stripe cladding layer 54 is formed on theetching stopper layer 52. This layer 54 may be a p-type In₀.5 Ga₀.15Al₀.35 P layer 54, doped with a p-type impurity such as Zn to aconcentration of 4×10¹⁷ atoms per cubic centimeter. The total thicknessof layers 20 and 54 is typically 0.8 micrometers. The layer 54 is formedby a crystal growth technique. The layer 54 serves as a part of theunderlying second cladding layer 20. A cap layer 56 is disposed on themesa cladding layer 54. The cap layer 56 may be a p-type In₀.5 Ga₀.5 Player that is approximately 0.05 micrometers thick and doped with ap-type impurity such as Zn to a concentration of 2×10¹⁸ atoms per cubiccentimeter. As shown in FIG. 6, these layers 54 and 56 have astripe-ridge mesa structure.

A current-blocking layer 58 is formed to cover the mesa structuresections 54 and 56. The current-blocking layer 58 may be a GaAs layer,typically 0.5 micrometers thick and doped with an n-type impurity suchas Si to a concentration of 2×10¹⁸ atoms per cubic centimeter. A planarshape of the current-blocking layer 58 is shown in FIG. 7. Thecurrent-blocking layer 58 has a stripe opening 60 in a central region.More specifically, the current-blocking layer 58 is not formed in aregion which is spaced apart inward from a laser beam output end of thetop surface of the stripe ridge mesa structure consisting of the layers54 and 56 by a predetermined distance d (=at least 5 micrometers). Thecurrent-blocking layer 58 covers only the exposed top surface of theetching stopper layer 52, slanted side surfaces of the stripe ridge mesastructure section, and both end portions (distance d=5 micrometers) ofthe mesa structure section. With such an arrangement, low-andhigh-energy band gap regions 18a and 18b are formed in the active layer18 in the same manner as in the above embodiment, as shown in FIGS. 8and 9.

An ohmic contact layer 62 (see FIG. 6) is formed on the current-blockinglayer 58 to bury the stripe opening 60 in the current-blocking layer 58.The ohmic contact layer 62 may be a p-type GaAs layer, approximately 3micrometers thick and doped with a p-type impurity such as Zn to aconcentration of 3×10¹⁸ atoms per cubic centimeter.

Also in the laser 50, an excellent NAM structure can be realized in thesame manner as in the laser 10 in the above embodiment. Morespecifically, a difference in complex index of refraction occurs betweenthe mesa-stripe cladding layer 54 and the current-blocking layer 58. Anoscillated laser beam is confined and guided by a ridge portion of themesa structure section on the basis of the difference in complex indexof refraction. A current blocked by the current-blocking layer 58 issupplied in a portion except for the terminal portions of the mesastructure section. The high-energy band gap region 18b in the activelayer 18 is "transparent" with respect to the oscillation lightwavelength determined in accordance with a band gap of the low-energyband gap region 18a. The reason for this is the same as in the aboveembodiment. As a result, undesirable self absorption near the lightoutput ends of the laser 50 could be suppressed or stopped, anddegradation in high-output laser oscillation could be eliminated.According to our experiments, high-level transverse mode stabilizedlaser oscillation having a light output of 50 mW or more could be stablymaintained.

FIG. 10 shows an InGaAlP NAM structure laser 70 according to stillanother embodiment of the present invention, which is generallydesignated by numeral "70". A semiconductor substrate 72 is a galliumarsenide (GaAs) layer of n-type conductivity. A first buffer layer 74 ofn-type conductivity is formed on the n-GaAs substrate 72. The bufferlayer 74 is, for example, a GaAs layer that is typically 0.5 micrometersthick and doped with an n-type impurity to a concentration of 8×10¹⁷ atoms per cubic centimeter. A second buffer layer 76 of n-typeconductivity is formed on the first buffer layer 74. The buffer layer 76may also be a In₀.5 Ga₀.5 P layer that is typically 0.5 micrometersthick and doped with an n-type impurity to a concentration of 8×10¹⁷atoms per cubic centimeter.

A double-heterostructure section is disposed on a surface of the secondbuffer layer 76. The double-heterostructure section consists of a firstcladding layer 78, an active layer 80, and a second cladding layer 82.The first cladding layer 78 may be an n-type In₀.5 Ga₀.15 Al₀.35 P layerhaving a thickness of approximately 0.8 micrometers, doped with ann-type impurity such as Si to a concentration of 4×10¹⁷ atoms per cubicCentimeter. The active layer 80 may be a nondoped In₀.5 Ga₀.5 P layer ofp-type conductivity, typically 0.06 micrometers thick. In principle, acarrier concentration of the active layer 80 is set to be 1×10¹⁷ percubic centimeter or less. The active layer 80 may be of an n- or p-type.The second cladding layer 82 may be a p-type In₀.5 Ga₀.15 Al₀.35 P layerhaving a thickness of approximately 0.8 micrometers, doped with a p-typeimpurity such as Zn to a concentration of 4×10¹⁷ atoms per cubiccentimeter.

The second cladding layer 82 is formed with a projecting portion (mesa)designated by numeral "84" in FIG. 10 by a known etching treatment. InFIG. 10, a waveguide channel region to which a current is mainlysupplied in an oscillation mode of the laser 70 is dot-ilustrated forthe purpose of visual distinction. A p-type cap layer 87 is formed onthe top surface of the mesa section 84. The cap layer 87 may be an In₀.5Ga₀.5 P layer that is doped with a p-type impurity such as Zn to aconcentration of 2×10¹⁸ atoms per cubic centimeter. An n-type GaAscurrent-blocking layer 86 is formed to bury side surfaces of the etchedportion, i.e., the mesa section 84, in the second cladding layer 82. Acomposition and formation conditions of the layer 86 may be the same asthose in the laser 50 in the above embodiment. The top surface of themesa section 84 is even with that of the current-blocking layer 86, asshown in FIG. 10. A p-type contact layer 88 is stacked on the layers 84and 86. A composition and formation conditions of the layer 88 are thesame as those of the laser 50 in the above embodiment. Metal layers 90and 92 are formed on the substrate 72 and the contact layer 88,respectively. The metal layers 90 and 92 serve as p-type n-typeelectrodes, respectively.

The above-mentioned laser structure 70 is manufactured as follows. Asshown in FIG. 11A, the layers 74, 76, 78, 80, 82, and 87 aresequentially formed on the substrate 72 by MOCVD crystal growth.Thereafter, a p-type In₀.5 Ga₀.15 Al₀.35 P layer 94 is disposed on thecap layer 87 by MOCVD crystal growth. The layer 94 serves as adamage-proof layer which has a great effect in the following etchingstep. Then, an SiO₂ masking layer 96 is deposited on the damage-prooflayer 94. The masking layer 96 has a stripe shape corresponding to aplanar shape of the above-mentioned mesa section 84.

Subsequently, selective etching is performed using the masking layer 96.The etching is performed halfway through the second cladding layer 82(the second cladding layer 82 is selectively etched), thereby formingthe mesa section 84, as shown in FIG. 11B. The mesa section 84 includesthe perfectly etched cap and damage-proof layers 87 and 94. During theetching step, the damage-proof layer 94 functions to reduce etchingdamages of the underlying layers 87 and 82. A solution mixture of ahydrogen bromide-based etching solution and hot sulfuric acid was usedas an etchant.

Thereafter, using the masking layer 96 again, the current-blocking layer86 is formed on both side surfaces of the partially etched secondcladding layer 82 and mesa section 84 by selective crystal growth usinga reduced-pressure MOCVD method, as shown in FIG. 11C. Prior to growthof the current-blocking layer 86, the masking layer 96 is subjected topartial etching, and the end portions are removed. Therefore, themasking layer 96 is treated to have an identical planar shape to thoseof the stripe openings 26 and 60 respectively shown in FIGS. 2 and 7.Therefore, the current-blocking layer 86 thus formed has an identicalrectangular shape to that in the case shown in FIG. 2 or 7. After thecurrent-blocking layer 86 is formed, the masking layer 96 was removedby, e.g., hydrogen fluoride. Then, the damage-proof layer 94 whichremained on the mesa section 84 was removed by hot sulfuric acid.

Subsequently, as shown in FIG. 11D, the contact layer 88 is formed onthe mesa section 84 and the current-blocking layer 86 by MOCVD crystalgrowth. Prior to growth of the layer 88, hydrogen selenide (H₂ Se),dimethylzinc (DMZ), or diethylzinc (DEZ) was flown together withphosphine (PH₃) at a crystal growth temperature (700° C.) for apredetermined time period, e.g., three minutes. When the additionalelectrodes 90 and 92 are formed in the resultant structure shown in FIG.11D by a known method, the InGaAlP laser is completed.

In the device 70 according to the third embodiment, in the same manneras in the above embodiments, an oscillation laser beam output end in theInGaP active layer 80 serves as a high-energy band gap region, thusproviding an excellent NAM structure. In addition, according to thisembodiment, the use of the InGaAlP layer as the damage-proof layer 94can allow the crystal growth process in a phosphine atmosphere to formthe layers 78, 80, 82, 86, 87, and 88 which are grown on the secondbuffer layer 76. This suggests a possibility of drastic reduction intime period required for the crystal growth process of the InGaAlP NAMstructure lasers in the present invention.

In addition, according to this embodiment, in the step of removing theSiO₂ layer 96 and the damage-proof layer 94 which is performed prior toforming the contact layer 88, hot sulfuric acid was selected as anetchant. Therefore, even if these layers 94 and 96 were carefullyremoved for a time period longer than that in a normal treatment,accidents wherein the underlying current-blocking layer 86 isundesirably etched or subjected to an etching damage did not occur atall. As a result, the thickness of the obtained current-blocking layer86 could be increased up to a desired value, and excellentcurrent-blocking characteristics could be obtained. The presentinventors have examined a cause and effect relationship (correlation)between the thickness of the damage-proof layer 94, a damage whichoccurs in practice, and a lateral etching amount upon formation of themesa section 84. As a result, the following relationships becameapparent. If the thickness of the InGaAlP damage-proof layer 94 was setto be 0.1 micrometer or less, the damage of the resultantcurrent-blocking layer 82 was noticeable. If the thickness of the layer94 was set to be 0.5 micrometers or more, a lateral etching amount uponformation of the mesa section 84 was increased. Therefore, the thicknessof the damage-proof layer must be selected within the range of 0.1 to0.5 micrometers.

FIG. 12 shows the entire structure of an InGaAlP NAM structure layeraccording to the fourth embodiment of the present invention, which isgenerally designated by numeral "100". The laser 100 includes an n-typeGaAs substrate 102. A first cladding layer 104 of an n-typeIn0.5(Ga_(1-x) Al_(x))₀.5 P layer, a non-doped InGaP active layer 106,and a second cladding layer 108 of a p-type In₀.5 (Ga_(1-x) Al_(x))₀.5 Player are sequentially formed on the substrate 102 to constitute adouble-heterostructure section. A p-type InGap cap layer 110 is formedon the second cladding layer 108. An n-type GaAs current-blocking layer112 and a p-type GaAs contact layer 114 are formed on the top surface ofthe cap layer 110. N-and p-side electrode layers 116 and 118 are formedon the substrate 102 and the contact layer 114, respectively. In FIG.12, the width (i.e., the waveguide channel width) of the stripe openingin the current-blocking layer 112 is denoted by reference symbol "Ws",and was set to be 7 micrometers in this embodiment. A length of thesubstrate 102 along the above channel width is denoted by referencesymbol "Ls", and was selected to be 300 micrometers. Although notvisible in FIG. 12, a resonance cavity length L of the laser 100 was setto be, e.g., 300 micrometers. The thickness of each of the first andsecond cladding layers 104 and 108 is denoted by reference symbol "h",and was set to be 0.8 micrometers. The thickness of the active layer 106is denoted by reference symbol "a", and was set to be, e.g., 0.06micrometers.

The values of the lengths Ls and Ws in the laser 100 are selected inconsideration of the following. The value Ls must be at least four timesthe value Ws for the following reason. A measurement result of changecharacteristics of a temperature increase T of the active layer 110 as afunction of the value Ls in a continuous oscillation mode of the laser100 is shown in the graph in FIG. 13. The temperature increase ΔT isobtained by repeating simulation calculation while the value Ls isvaried using a thermal conduction model. As is apparent from the graphin FIG. 13, the temperature increased ΔT is varied depending on thesubstrate length Ls with a consistent tendency. The temperature increaseΔT must be minimized. If a target value of the temperature increase ΔTis 10 degrees, and measurement values of the substrate length Ls withrespect to the channel width Ws are plotted, the graph in FIG. 14showing correlation characteristics can be obtained. Assume that, inFIG. 14, a solid curve represents actual characteristics, and that abroken line is a basic line representing "Ls=4.Ws". When a region inwhich ΔT≦10 is extracted from the graph in FIG. 14, the length Ls isalways four times or more the channel width Ws. On the basis of theconsideration based on the above fact, in the laser 100 of thisembodiment, when the channel width Ws was set to be 7 micrometers, thesubstrate length Ls was selected to be 300 micrometers.

Although the invention has been described with reference to specificembodiments, it shall be understood by those skilled in the art thatnumerous modifications may be made that are within the spirit and scopeof the invention.

In the above embodiments, the internal current-blocking structure laserand the transverse mode stabilized structure laser having a light guidestructure of a stripe ridge cladding layer are exemplified. However, asemiconductor laser having a similar structure near a light output endcan achieve laser characteristics which are not degraded up to a highoutput, as a matter of course. The cause of a difference in band gapenergy, which in turn is caused by a difference in atomic alignmentdepending on a crystal growth method and growth conditions, isconsidered as follows. The atomic distances between In, Ga, and Al(Group III) and P (Group V) are different from each other. The atoms arealigned not randomly on the Group III lattice points but regularly inaccordance with the crystal growth process, thus setting a stable statewith low free energy. Therefore, it is considered that such a phenomenonmay occur in not only InGaAlP, but also a general mixed crystal (e.g.,InGaAsP, InGaAlAs, or ZnSSe) of compound semiconductors having differentlattice constants. For example, the above embodiments can be applied toan active layer having a super-lattice structure of GaAs or AlGaAs.Therefore, the above embodiments can be applied to a semiconductor laserusing the above compound semiconductor mixed crystal except for InGaAlP.Various changes and modifications may be made without departing from thespirit and scope of the present invention.

What is claimed is:
 1. A method of manufacturing a semiconductor laser,comprising the steps of:forming a double-heterostructure section ofindium, gallium, aluminum, and phosphorus above a semiconductorsubstrate by a first metal organic chemical vapor deposition process,said double-heterostructure section including first and second claddinglayers and an active layer sandwiched therebetween; forming a firstsemiconductor layer on said double-heterostructure section; selectivelyetching said semiconductor layer to form a stripe opening in saidsemiconductor layer; and forming a second semiconductor layer on saidfirst semiconductor layer to cover said opening by a second metalorganic chemical vapor deposition process, whereby a band gap energy ofsaid active layer is reduced in a first region located immediately belowsaid opening, and a band gap energy of a second region except for saidfirst region is increased to be higher than the band gap energy of saidfirst region, said second region including a peripheral endcorresponding to a light output end portion of said laser, so that selfabsorption of a laser beam at said light output end portion in anoscillation mode of said laser is suppressed.
 2. The method according toclaim 1, wherein a crystal growth temperature in the second metalorganic chemical vapor deposition process is lower than that in thefirst metal organic chemical vapor deposition process.
 3. The methodaccording to claim 2, wherein said first semiconductor layer comprisesgallium arsenide.
 4. The method according to claim 3, wherein said firstand second semiconductor layers serve as current-blocking and contactlayers, respectively.
 5. The method according to claim 4, wherein saidsecond cladding layer is subjected to selective etching prior toformation of said first semiconductor layer, thus forming a stripeprojection layer.
 6. The method according to claim 5, wherein said firstsemiconductor layer covers both side surfaces and both peripheral endsof said projecting layer.
 7. The method according to claim 6, whereinsaid active layer comprises indium, gallium, and phosphorus.
 8. A methodfor manufacturing a semiconductor laser comprising the steps of:forminga first cladding layer above a semiconductive substrate; forming anactive layer on said first cladding layer; forming a second claddinglayer on said active layer, said second cladding layer including awaveguide channel region to which a current is mainly supplied in anoscillation mode of said laser, and an output end from which anoscillated laser light is externally output; forming, on said secondcladding layer, a current-blocking layer having an opening defined abovea central region of said waveguide channel region, for covering saidoutput end of said second cladding layer; and defining a specific energyband gap region in said active layer at a peripheral portion adjacent tosaid output end of said second cladding layer, said region including afirst energy band gap larger than a second energy band gap in asubstantially central region of said active layer, thereby to prevent orsuppress self absorption of the oscillated laser light at said outputend of said second cladding layer.
 9. The method according to claim 8,wherein said active layer comprises indium, gallium, and phosphorus. 10.The method according to claim 9 wherein said first and second claddinglayers comprise indium, gallium, aluminum, and phosphorus.
 11. Themethod according to claim 10, wherein an elongated projecting layerserving as said waveguide channel region is formed in said secondcladding layer.
 12. The method according to claim 11, wherein saidopening of said current-blocking layer is formed so as to be elongatedand correspond to said projecting layer.
 13. The method according toclaim 10, wherein said waveguide channel has a selected width, and alength of said substrate in a direction perpendicular to said channelregion is at least four times said selected width of said waveguidechannel.